1. Field of the Invention
The present invention relates to a solid-state image converting device and particularly to a solid-state image converting device converting a radiation image of a non-visible wavelength such as an infrared ray, an X-ray and a .gamma.-ray into a visible light image.
2. Description of the Related Art
In general, as shown in FIG. 1, a solid-state image converting device has the structure in which a transparent substrate 1, a transparent electrode 2, an electroluminescent layer 3, a reflecting layer 4, a non-transmitting layer 5, a photoconductive layer 6 (hereinafter called as a PC layer), and a back electrode 7 are sequentially laminated. This type of the solid-state image converting device converts the non-visible wavelength image into the visible image by the following principle.
An X-ray image is radiated from the side of the back electrode 7 in a state that an alternating voltage of 400 to 500V is applied between the transparent electrode 2 and the back electrode 7. In a state that no X-ray image is radiated, the electroluminescent layer 3 is light emitted since most of the applied voltage is applied to the PC layer 6 having high resistivity of tens of M.OMEGA.cm. However, when the X-ray image is radiated, the resistivity of the PC layer 6 is decreased in accordance with the increase in exposure does of the X-rays. Due to this, a part of the applied voltage is distributed to the electroluminescent layer 3. Then, the electroluminescent layer 3 exceeds the starting voltage for light-emission, thereby a converted output image can be obtained on the side of the transparent substrate 1. At this time, the reflecting layer 4 and the non-transmitting layer 5 are provided in order to prevent optical feedback applied to the PC layer 6 from the electroluminescent layer 3. That is, these are provided to prevent the image from being not formed on the side of the transparent substrate 1 because of the fact that light generated by the electroluminescent layer 3 is returned to the PC layer 6.
In the above-structured image converting device, the layers other than electrodes 2 and 7 are formed by dispersing powder material in an organic binder and applying the dispersion by a screen printing and the like. The layer structure, which is formed by such a method, is generally called as a dispersion type. The film thickness normally ranges from tens of .mu.m to hundreds of .mu.m. Therefore, the above structured solid-state image converting device is called as a dispersion-type solid-state image converting device. The dispersion-type solid-state image converting device, however, has the following problems.
(1) Uniformity of a light emitting surface is determined depending on mainly the size of particles of phosphor constituting the electroluminescent layer 3, the dispersing state, and the applying film thickness.
(2) Since the film thickness of the PC layer 6 is large (hundreds of .mu.m), a large applied voltage is needed.
(3) Since the electroluminescent layer 3 is the dispersion type, its luminance is low and its life time is short.
(4) Since the electroluminescent layer 3, the reflecting layer 4, and the non-transmitting layer 5 are provided, the radiation ray from the side of the transparent substrate 1, is not transmitted.
(5) Since the thin film, which is formed by depositing metal, is used as the back electrode 7, an excitation efficiency of the PC layer 6 by the radiation ray from the back side is low.
Particularly, due to problems (4) and (5), in an actual situation, only a solid-state X-ray image converting device having high transmissivity to the back electrode 7 is put into practical use at present.